Generally, a bus is a type of vehicle that is designed to carry and move a large number of passengers compared to a more compact vehicle. A typical bus includes a front end with the windshield for providing a view of the road ahead, two sidewalls, and a rear end in which the engine is accommodated, a floor, and a roof. The roof includes an outer roof surface and a roof frame structure. The roof frame structure may have a plurality of roof bow members and a plurality of roof stringers, and the outer roof surface may cover the roof frame structure. Recently, low-floor buses have been introduced. Compared to conventional buses, the low-floor bus has a flat and lower floor, eliminating steps at an entrance(s) of the bus, thereby improving accessibility for users such as the elderly, people with disabilities, and pregnant women.
However, as the floor of the low-floor bus is lowered, a fuel tank such as CNG bombe may be disposed on the roof of the bus. Thus, the low-floor bus has a high center of gravity, and the risk of rollover may increase. With the addition of stiffeners or reinforcing materials to reinforce the roof frame structure, the weight of the low-floor bus may increase by approximately two tons, compared to that of a typical bus. For example, the fuel economy of the low-floor bus may be approximately 1.7 km/m3, which is reduced by roughly 23%, compared to approximately 2.2 km/m3, the fuel economy of the typical bus. In addition, as the low-floor bus includes a substantial amount of stiffeners or reinforcing materials added to the roof frame structure, the manufacturing and assembly thereof may be complex, and thus the manufacturing costs thereof may be increased. For example, the cost of the low-floor bus may be approximately 210 million won, which is greater than twice the cost (e.g., approximately 100 million won) of the typical bus.